Change string representation of dtypes to short version when available.

[scott-griffiths]

For __str__ and __repr__ I now prefer the shorter dtype names, and I'd also like them to be used in the bitformat project.

It's possible that some code might depend on the string representation of the data type, but frankly it shouldn't (unless it's my own test code!) so I don't consider this a backwardly incompatible change.

[scott-griffiths]

No longer needed as bitformat doesn't have bitstring as a dependency any more.